While polyesterurethanes prepared by the reaction of a hydroxyl terminated ester and p-phenylene diisocyanate are known, these materials are not of substantial commercial importance. One of the problems in making such materials, particularly on a continuous basis, is the instability of p-phenylene diisocyanate in the molten state that is required for continuous operations. The melting point of p-phenylene diisocyanate is about 95.degree. C. compared to the melting point of about 37.degree. C. for diphenylmethane-p,p'-diisocyanate, for example, that is often used in commercial preparation of polyesterurethanes. At 95.degree. C., the rate of degradation of the p-phenylene diisocyanate is excessive and the degradation is evidenced by the formation of dimers, trimers, and higher oligomers at the rate of about 0.33% per hour. These degradation products do not melt below about 200.degree. C. so once they are formed in storage tanks or metering system lines, they tend to plug the equipment and are difficult to remove. Stabilizers for the degradation of p-phenylene diisocyanate at elevated temperatures are not known. An improved method for the preparation of polyesterurethanes with p-phenylene diisocyanate that avoids these problems is desired.